Summary Human germline cells are essential for human reproduction as only these cells are capable of differentiating into gametes and transmitting DNA from parent to child. The pioneering cells of the human germline begin to form during prenatal life when a small number of embryonic cells are set aside around the time of embryo implantation and gastrulation in a process known as human primordial germ cell (hPGC) specification. This critical event in human germline cell development has a tremendous impact on an individual's future reproductive health as a failure in hPGC specification causes certain infertility. In this competitive renewal, the goal is to increase our fundamental knowledge on the cell and molecular basis of hPGC specification. Based on experimental results in the previous funding period, we aim to use human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) and the differentiation of hPGC-like cells (hPGCLCs) to achieve this goal. The overall hypothesis is that non-rodent and human-specific molecular events have evolved to regulate hPGC specification. Given that the focus of this grant is largely on regions of the genome that are uniquely human, this project is perfectly suited to the use of human cell-based models. In aim 1, the hypothesis to be addressed is that TFAP2C-bound human-specific retrotransposons regulate hPGC specification. In aim 2, the hypothesis to be addressed is that the expression of TFAP2C bound retrotransposons are regulated by targeted changes to the epigenome during hPGCLC differentiation. In the third aim, we will evaluate the relationship between TFAP2C and SOX17 in hPGC specification, with the hypothesis that TFAP2C functions upstream of SOX17 in a lineage primed hPGC progenitor to regulate specification of hPGCs. In summary, this competitive renewal builds upon success from the first funding period to contribute essential knowledge on the identification of new loci in the human genome that have evolved to regulate the specification and identity of hPGCs.